Insulated electrical conductor



Dec. 11, 1934. B, H, REEVE "1;9s3,5s3

INSULATED ELECTRICAL CONDUCTOR Filed April 21, 1932 4566/05 Bra/a Sfranded (anducfar Mam/Med (ambn'c lead Men M ead Jbenf/l Fe/fed Awe; fos (on fw'n 171g GWson/le H andffi/ari lafed P/Iena/ INVENTOR Beau/0rd IL Ieere 8 ATTORNEY Patented Dec. 11, 1934 PATENT OFFICE I 1,983,563 INSULATED ELECTRICAL CONDUCTOR Beauford H.

Reeves. New Haven, to Bockbatoa Products Conn, IIIIMI' p rati n. a corpo'naon of Massachusetts Application April :1, 1002, Serial No. 000,017 10 Claim. (01. 113-20 This invention relates to insulated electrical conductors and has for its object the provision of an improved article of this character. The invention is especially directed to the provision of an improved electric cable for low voltage power networks'and contemplates for such purposes an electric cable having effective inherent arcquenching characteristics upon the occurrence of a short circuit or similar fault.

In my copending United States patent application, Serial No. 550,063, filed July 11, 1931, I have disclosed an insulated electrical conductor in which effective arc-quenching characteristics are obtained by associating with the insulation surrounding the conductor a compound which when heated, by the occurrence of a short circuit or similar fault, evolves a considerable volume of arc-extinguishing gases. From an arc extinguishing standpoint, urea has been found a most advantageous compound for the purpose. But in the course of my investigations, I have discovered that while the dielectric strength of urea. is satisfactory at ordinary or moderate temperatures, its dielectric strength rapidly decreases at. higher temperatures, so that at temperaturesv of 90 C. and higher urea is practically a conductor of electricity. I have also found that urea at elevated temperatures, say above 170 C., has a decided corrosive action on copper wire. Further, I have found that the relatively low melting temperature of urea (about 132 C.) is conducive, particularly in vertically disposed cables, to the escape of molten urea at a time and place where its arc-quenching action is needed. By the present invention these difliculties in the use as arc-extinguishing agents of urea and similarly behaving compounds are effectively overcome.

In its broad aspect, the present invention involves protecting the electrical conductor from actual contact with. the arc-extinguishing agent, such as urea, associated therewith. protection is conveniently accomplished by interposing between the conductor and the arc-extinguishing agent one or more layers of material of high dielectric strength and impervious to molten or equivalent agent. Such material may be varnished 'cambric of high dielectric strength, or other appropriately treated or impregnated insulating substance. If of relatively low melting temperature, such as urea, the arc extinguishing agent is preferably admixed with some substance capable of increasing the coherence between the molten particles thereof. Cornstarch has been found satisfactory for this purpose.

In the accompanying drawing,

Fig. 1 is an insulated conductor embodying the invention with the various insulating layers exposed, and t Fig. 2 is a slightly modified form of the invenion.

In the form of the invention illustrated in Fig. 1, the metallic conductor element, usually in the form of a stranded cable, is surrounded by one or more layers of varnished cambric tape, wound spirally around the conductor-with edges overlapping. Varnished cambric possesses relatively high dielectric strength and is not affected by molten urea. It therefore provides a suitable barrier both from the standpoint of protecting the conductor, as well as supplying the requisite dielectric strength under conditions of elevated temperature.

In order to assure a tight-closed joint where the varnished, cambrlc tape overlaps, I have found it advisable to apply a thin layer of felted asbestos insulation directly to the conductor before wrapping on the varnished cambric tape. This inner layer of asbestos insulation serves to provide a smooth, circular cross-section on which to wrap the tape, and at the same time has the additional advantages of acting as a heat-barrier between the conductor and the varnished cambric tape in the event of abnormal heating of the conductor as a consequence of over-loads.

The varnished cambric layer is surrounded by a layer of felted asbestos containing or impregnated with urea or equivalent arc-quenching agent or compound. The conductor and its associated insulating layers are preferably enclosed by a protective covering of asbestos braid, or the like, to prevent cracking of the underlying insulating layers on bending of the conductor. The asbestos braid may or may not, as desired, be treated with the arc-quenching agent. The insulated conductor is finally provided with the usual lead sheath, or equivalent outer protective covering.

In the modification of the invention illustrated in Fig. 2, the varnished cambric layer of the construction of Fig. 1 is replaced by an insulating medium. such as felted asbestos appropriately treated to increase its dielectric strength and to render it impervious to molten urea. The metallic conductor, such as a stranded cable, is first covered with a felted wall of asbestos fibers, applied in roving form, say to a wall thickness of about 0.030 inch. It is now my preferred practice to impregnate this felted wall of asbestos with a compound consisting of 40% chlorinated phenol and 60% gilsonite. I have secured very satsuch temperature.

'ly saturate. the asbestos.

2 isfactory results in practice with ordinary gilsonite which is solid at room temperature and with a chlorinated phenol which is a syrupy liquid at The gilsonite is first melted and the chlorinated phenol is added thereto and homogeneously admixed therewith at a temperature around 200 C. The conductor with its first covering of felted asbestos is passed through this mixture, at a temperature of about 200 C., allowing sufficient time for the mixture to thorough- As the conductor emerges from the mixture, it is squeezed by revolving springs so as to compress the insulating wall and wring out the excess of the mixture. The conductor then passes through revolving dies which further compress and smooth the covering. By the time the conductor passes through these dies, the mixture has cooled suficiently to make it relatively solid and non-sticky, and the conductor is wound up on a reel.

The second operation consists in applying another wall of felted asbestos over the first treated wall, and impregnating the second wall with the arc-quenching agent, such as urea. This second wall of asbestos may advantageously be of approximately the same thicmess as the first wall.

I now prefer to make up the arc-extinguish-= ing agent or compound of 200 parts of urea, 4 parts of ordinary corn-starch and parts of water, by weight, and heat to a temperature of to C. The compound is maintained at this temperature while the conductor with its covering of asbestos is passed therethrough at the proper speed to allow complete penetration of the second felted asbestos wall. On emerging, the conductor is again subjected to the wringing and smoothing operations previously described. It is then wound on a reel and placed in a drying oven for about 24 hours at F. to drive off excess water.

The conductor is now covered with a tightly woven braid of asbestos yarn, say approximately 0.045 inch in thickness, and is again passed through the arc-extinguishing compound at a temperature of about 1l0-120 (3., and again passed through the revolving spring wipers and the polisher. It is then again dried to remove moisture, and finally covered with the usual lead sheath to form the completed cable.

It will be understood that the layer of felted asbestos surrounding the varnished cambric, as well as the layer of asbestos braid if desired, in the construction of Fig. 1, may be impregnated with urea, or other appropriate arc-extinguishing agent or compound, wrung, polished and dried in the same manner as described in more detail in connection with Fig. 2.

Urea, at elevated temperatures, such as prevail upon the occurrence of a short-circuit or similar fault, evolves large volumes of arc-extinguishing vapors or gases, consisting mainly of ammonia and carbon dioxide. Since these gases are noncombustible, non-explosive and non-toxic and are evolved in very effective volume, urea is an outstanding arc-quenching agent in the practice of the present invention. The inclusion of cornstarch, wheat fiour or equivalent starch-like substance, renders the resulting mixture with urea non-flowing at temperatures up to 150 C., which is considerably above the melting point of urea (132, C.) The action of the starch is to increase thecohesion between the particles of the urea, and to produce a jelly-like consistency instead of a liquid at temperatures up to 150 C. This is of particular advantage in long vertical runs of cable.

The function of the chlorinated phenol and the gilsonite impregnant in the first layer of asbestos (Fig. 2) is to produce a covering next to the conductor which is impervious to melted urea and at the same time provides suiiicient dielectric strength to give ample factor of safety for low voltage use under all conditions of operating temperatures. Arc temperatures will, of course, break down the compounds in all parts of the insulation but the compounds are such that no toxic, inflammable, or explosive gases are evolved. For certain types of conductors, the gilsonite may be replaced by an equivalent amount of chlorinated naphthaline, which is solid at ordinary room temperature.- The dielectric strength of chlorinated naphthaline deteriorates at elevated temperatures, and while it is effective within a certain range of temperatures, gilsonite is generally preferable.

Insulated electrical conductors embodying the invention, for example, of the construction illustrated in Figs. 1 and 2, have stood up satisfactorily after baking for 90 hours at 144 C. The dielectric strength when hot and after cooling was sufiiciently high to provide ample factor of safety. In actual operating practice, upon the occurrence of a short-circuit or similar fault, the effective volume of gas evolved from the arc extinguishing agent or compound serves to promptly suppress any are with a minimum of damage to the conductor itself and its surroundings.

I claim:

1. An insulated electrical conductor comprising a metallic conductor surrounded by a layer of asbestos impregnated with chlorinated phenol and gilsonite, and a layer of asbestos impregnated with urea surrounding said first mentioned layer of asbestos.

2. An insulated electrical conductor comprising a metallic conductor surrounded by a layer of asbestos containing chlorinated phenol, and a layer of asbestos impregnated with urea surrounding said first mentioned layer of asbestos.

3. An insulated electrical conductor comprising a. metallic conductor surrounded by an insulating medium containing chlorinated phenol and gilsonite, and a second insulating medium surrounding said first mentioned insulating medium and containing a compound capable at elevated temperature of evolving an arc-extinguishing gas in efiective volume.

4. An insulated electrical conductor comprising a metallic conductor surrounded by an insulating medium containing chlorinated phenol and gilsonite, and a second insulating medium surrounding said first mentioned insulating medium and containing urea.

5. An insulated electrical conductor comprising a metallic conductor surrounded by a layer of asbestos containing urea, and an insulating medium containing chlorinated phenol and gilsonite interposed between the conductor and said layer of asbestos. I

6. An insulated electrical conductor comprising a metallic conductor surrounded by a layer of felted asbestos containing chlorinated phenol and gilsonite, a second layer of felted asbestos containing urea and surrounding said ,first-mentioned layer of asbestos, and a. layer of woven asbestos containing urea surrounding said second layer of asbestos.

7. An insulated electrical conductor comprising a metallic conductor surrounded by an insulating medium containing urea admixed with a starch-containing substance capable of increasing the cohesion between the particles of urea at temperatures above the melting point 01' urea.

, 8. An insulated electrical conductor comprising a metallic conductor surrounded by an insulating medium containing urea admixed with comstarch. v

9. An insulated electrical conductor comprising a metallic conductor surrounded by an intemperatures above the melting point of urea, and a layer of high dielectric strength and impervious to molten urea interposed between the conductor and said insulating medium.

10. An insulated electrical conductor comprising a metallic conductor surrounded by a layer of asbestos impregnated with urea admixed with corn-starch to increase the cohesion between the particles of urea at temperatures above the melting point of urea, and a layer of high dielectric strength and impervious to molten urea interposed between the conductor and said layer of assulating medium containing urea admixed with has a starch-containing substance capableol increasing the cohesion between the particles of urea at BEAUFURD H. REEVES. 

